It is customary in miners' cap lamps to provide a backup source of light in addition to the primary source of light used by the miner in the performance of his duties. This backup source may take the form of a second bulb, where the primary source is a bulb of single-filament construction, or of a second filament in the same bulb envelope as a primary filament. In either case it is desirable that the backup source may be used for the performance of normal duties should the primary light source or filament fail.
Certain countries specify that the wattage of the backup light source, as determined by its current draw, be substantially less than that of the primary source. As an example, a primary source may have a 4.8 watt filament, and a secondary source a 3.2 watt filament. In this case, should the miner be trapped, he will switch to the secondary source, thereby prolonging the duration of available light at the expense of intensity. This approach carries the disadvantage of reduced light output and thus reduced visibility for the miner under normal working conditions should the primary source fail.
A second approach, which has not met with significant acceptance, involves switching a resistance element into and out of series connection with either the primary or the secondary filament. Although this produces the desired battery conservation effect, it is not efficient in terms of conversion of battery power to light since some of the power available from the battery is consumed and dissipated as heat by the resistance element.
A third approach is to provide switching means for connecting the two filaments in series, and operating the lamp in this mode in an emergency situation. This approach carries the disadvantage of reliance upon the integrity of both filaments; should one fail, the miner no longer has recourse to low light intensity operation.
Any system which is to be used in a mining application must also meet certain physical and safety requirements, e.g. the miner must never be allowed access to any live electrical conductors, any switch means must be protected against physical damage, and any entry point into the lamp structure must be adequately sealed against entry of water and dust.
To date, no proposed system has successfully dealt with the problems as described above in an effective and efficient manner.